Small toom43 cleanup

[Torbjorn Granlund]

nisse at lysator.liu.se (Niels Möller) writes:

  I've adapted toom32_mul to use a proper itch/scratch interface. I also
  simplified the allocation by requiring that s + t >= 5, so that 5
  temporary values of n+1 limbs each fit in the product area. That
  should be true except for very small inputs (I think an >= 25 or bn >=
  19 is sufficient).
  
  Marco, Torbjörn, does this look ok?
  
It is OK with me.  

-- 
Torbjörn